Plant water regime

1. Plant water regime • Transport of liquid water • Transport of water across membranes • Absorption of water by roots • Radial transport of water in roots • Root pressure • Absorption of water by shoots

2. Basic characteristics of water transport on cellular level • Transport of liquid water across membrane • (plasmalemma, tonoplast, organelle membranes) • Jw = Lpw = Lp (p + s) • Jw - membrane water transport rate, Lp - membrane water permeability, • w - water potential difference, p - pressure potential difference, •  - reflection coefficient, s - osmotic potential difference between two membrane sites

3. Coupling of water and solute flow • Thermodynamic of irreversible processes, equation for water and single solute flow • Jw = Lww + Lwss • Js = Lss + Lsww • Jw -transport of water, Js - transport of solute, w - gradient of chemical potential of water, s - gradient of chemical potential of solute, Lw, Ls - membrane permeability for water and solute, respectively, Lws = Lsw (Onsager reciprocity coefficients)

4. Interactions between water and solute during membrane transport

5. Membranes, aquaporins • Membrane: phospholipid bilayer (transport of O2, CO2) + transmembrane proteins (transport of water, ions including protons, some hydrophylic organic substances); diffusion and active transport (ATPase, carriers, antiport, symport) • Aquaporins belongs into "major intrinsic proteins" (MIP); 25 - 30 kDa • tonoplast intrinsic proteins (TIP), plasmalemma intrinsic proteins (PIP) • passive transport in both directions, water permeability 10 to 100 times higher than that of phospholipid bilayer; widespread occurrence, high heterogeneity inside plant and among plant species (Arabidopsis35 TIP or PIP genes)

6. Aquaporins

7. Aquaporins (Luu and Maurel 2005)

8. Aquaporins • Regulation of water transport by changes in occurrence (gene expression) or by changes in permeability • phosphorylation increases permeability, dephosphorylation decreases permeability • Ca2+ • Na+, Cl- • lack of O2, ROS • water stres, ABA • circadial rhythm • different selectivity • Importance for water redistribution inside cells (much more TIP than PIP), and rapid transport between neighbouring cells (stomata, motoric cells, elongation growth, under decreased apoplast transport)

9. Water uptake Jw = Lsrw Jw - absorption rate, Lsr - conductance in the soil-root boundary, w - difference in water potential between soil and root Ma = A × Lsrw Ma - amount of water absorbed by the roots, A - exchange area of absorption zone Availability of soil water is dependent on amount of water, amount of solutes, (osmotic potential of soil solution), size of capillary pores (matric potential of soil).

10. Adaptations of roots for sufficient water uptake • Hydrotropic growth – slower growth on the side where is higher soil moisture and more rapid growth on the opposite side. • Root distribution – species specific (shallow, deep, ormultilayer root system) • Anatomical adaptations – endodermis, exodermis, etc. • Physiological adaptations – e.g. osmotic adjustment • Soil water potential is mostly less negative than root water potential, but under special conditions it might be more negative (transport of water from plant to soil). • Plant - lift for water

11. Root morphology and anatomy • Most rapid absorption is in the apical part of the root (5 - 10 cm) where is the highest occurrence of root hairs. • Number of root hairs is huge, their longevity short (max. several weeks). They increase absorption area considerably. • Water absorption by older (suberized) root parts is much slower. Nevertheless, due to large area the amount of water might be about 30%. • Arbuscular mycorrhiza or ectomycorrhiza • Great variability in root anatomy

12. Radial water transport in root • Three pathways: • apoplast (cell walls, intercellular spaces) • symplast (cell protoplasm connected by plasmodesmas) • across cells • In water transport (Jr), both the gradient of pressure potential (p) and the gradient of osmotic potential (s) take part, their importance is dependent on transport pathway and transpiration rate. • Jr = Lrw = Lr (p +s) • Lr- root conductance,  - reflection coefficient

13. Water transport pathways

15. Transport pathways • Apoplastic pathway - under high p and high transpiration rate, its conductance is high, reflection coefficient is low (mass flow of water and solutes), it is limited by endodermis and exodermis • Symplastic pathway + across cells - s is important, reflection coefficient is high, its conductance is rather low and dependent on aquaporins, ABA, Ca2+ • Similar transport mechanism: radial transport in stem, longitudinal transport in root and stem tissue with exception of vascular system, transport in leaf tissue • Transport of water is dependent on: 1) morphology and anatomy, 2) transport mechanism 3) water-solute interactions, 4) activity of aquaporins

16. Root pressure • Under high soil moisture and high air humidity (low transpiration rate) • Important for vein filling in spring or after embolism • Gutation, exudation • Mechanism: transport osmotically active compounds into xylem creates gradient of osmotic potential between xylem and root cells which is driving force for water transport into xylem

17. Model of radial transport of water and ions

18. Water uptake by shoot • The water source: rain, dew, fog, air humidity near 100 % • Under rather low water potential in shoot, dependent also on leaf wettability • For plant water balance is usually more important limitation of transpiration than water uptake • Epiphytic plants (Bromeliaceae, Orchidaceae)